It is well known to one skilled in the art that an unsaturated hydrocarbon compound can be produced by a thermal cracking process. For example, a fluid stream containing a saturated hydrocarbon such as, for example, ethane, propane, butane, pentane, naphtha, or combinations of any two or more thereof can be fed into a thermal (or pyrolytic) cracking furnace. Within the furnace, the saturated hydrocarbon is converted to an unsaturated hydrocarbon compound such as, for example, ethylene and propylene. Unsaturated hydrocarbons are an important class of chemicals that find a variety of industrial uses. For example, ethylene can be used as a monomer or comonomer for producing a polyolefin. Other uses of unsaturated hydrocarbons are well known to one skilled in the art.
However, an unsaturated hydrocarbon produced by a thermal cracking process generally contains an appreciable amount of highly unsaturated hydrocarbons such as the less desirable alkyne(s), diolefin(s), polyene(s), or combinations of two or more thereof. For example, ethylene produced by thermal cracking of ethane is generally contaminated with some acetylene which must be selectively hydrogenated to ethylene, but not to ethane, in a hydrogenation reaction. Similarly, propylene produced by thermal cracking of a saturated hydrocarbon is generally contaminated with propyne and propadiene which must be selectively hydrogenated to propylene, but not propane. In a thermal cracking process for producing a butene, butynes and butadienes are generally co-produced which must be selectively hydrogenated to a butene, but not a butane.
The so-called pygas which is a fluid stream containing hydrocarbons having 5 or more carbon atoms per molecule comprises debutanized aromatic concentrate (hereinafter referred to as DAC). Pygas or DAC therefore comprises a large variety of, for example, pentynes, pentadiene, pentatrienes, hexynes, hexadienes, hexatrienes, aromatic compounds such as, for example, benzene, toluene, xylenes, styrene, and ethylbenzene. In other words, pygas or DAC comprises a mixture of highly unsaturated C.sub.5 + hydrocarbons, i.e., hydrocarbons containing 5 or more carbon atoms per molecule. Generally, the trienes are hydrogenated to dienes (diolefins) which in turn are selectively hydrogenated to monoolefins, but not to alkanes. For an aromatic compound, such as styrene which is hydrogenated to ethylbenzene, the aromatic ring is not affected by the selective hydrogenation.
In all cases, the highly unsaturated hydrocarbons described above are undesirable because they are high reactive and tend to polymerize thereby forming gums if they are left in the product stream which is used for gasoline or for further processing. As such, they must be removed. A preferred process for removing the highly unsaturated hydrocarbons is a selective hydrogenation which is defined hereinbelow.
The selective hydrogenation of a highly unsaturated hydrocarbon is generally, commercially carried out in the presence of an alumina-supported palladium catalyst. In the case of the selective hydrogenation of acetylene to ethylene, a palladium-containing catalyst supported on an alumina in which palladium is optionally distributed on the skin of the aluminum support can be employed. See for example U.S. Pat. No. 4,404,124 and U.S. Pat. No. 4,484,015, disclosures of which are herein incorporated by reference. The operating temperature for this hydrogenation process is selected such that essentially all highly unsaturated hydrocarbon, such as, for example, acetylene is hydrogenated to its corresponding alkene such as, for example, ethylene thereby removing the alkyne from the product stream while only an insignificant amount of alkene is hydrogenated to alkane. Such a selective hydrogenation process can minimize the losses of desired unsaturated hydrocarbons and, in the front-end and total cracked gas processes, avoids a "runaway" reaction which is difficult to control, as has been pointed out in the above-identified patents.
It is generally known to those skilled in the art that impurities such as carbon monoxide, H.sub.2 S, COS, mercaptans, organic sulfides, thiophenes, or derivatives thereof which are present in a product stream such as, for example, pygas can poison and deactivate a palladium-containing catalyst. For example, carbon monoxide is well known to temporarily poison and deactivate such a hydrogenation catalyst. There is therefore an ever-increasing need to develop a catalyst which is suitable for selective hydrogenation of a highly unsaturated hydrocarbon such as, for example, a diolefin in a pygas, especially in the presence of an impurity, to a monoolefin.
Palladium supported on alumina has been successfully used in dry hydrogenation processes for many years. However, in some processes such as the so-called "total cracked gas" process in which the steam is not removed from the olefins stream, the selective hydrogenation of an alkyne to alkene must be accomplished in the presence of steam. In such processes, the alumina supported catalyst may have a much shorter life because alumina is not stable in steam. Therefore, there is also an increasing need to develop a palladium catalyst on a steam-stable support.
As such, development of an improved palladium catalyst and a process therewith in the selective hydrogenation of a highly unsaturated hydrocarbon such as a diolefin to a monoolefin in the presence of an impurity would be a significant contribution to the art and to the economy.